1. Field of the Invention
The present invention relates generally to a mobile communication system and base station identifier management method thereof and, in particular, to a mobile communication system including macro and pico base stations and method for managing the base station identifiers in order to avoid Physical Cell Identifier (PCI) confusion among the pico base stations in the mobile communication system.
2. Description of the Related Art
Mobile communication systems are evolving to provide users with various services utilizing a high data rate. In order to provide these services, the sizes of cells in mobile communication systems tend to increase for system capacity maximization, while the use of the pico base station increases in office/home environments. With the widespread use of the pico base stations, it has become necessary to continue network optimization for frequent installation and uninstallation. This necessity causes an increase in system maintenance and management costs, and therefore, system auto-installation and self-organization network technologies appear to reduce the system maintenance and management cost. Such system auto-installation and self-organization network technologies are being standardized in the standardization organizations such as the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) and the Institute of Electrical and Electronics Engineers (IEEE).
In order to discriminate among the cells per base station, a Physical Cell Identifier (PCI) in an LTE system may be used. An operation where a terminal performs handover between base stations using the PCI in an LTE system is described as follows.
A base station receives a PCI of the cell having the best signal strength among the neighbor cells from the terminal that is required to perform handover. The base station searches a Neighbor Relation Table (NRT) for the cell corresponding to the PCI to determine it as the handover target cell. Next, the base station sends the context information of the terminal to the target cells to perform the handover.
However, if a plurality of pico base stations exist within the range of a macro cell managed by the macro base station, there can be multiple cells that have the same PCI. This repeated use of the same PCI by multiple neighbor cells leads to a condition referred to as PCI confusion, in which the normal handover becomes impossible. In this case, the source base station to which the terminal is connected performs requests the terminal to report an Evolved Universal Mobile Telecommunications System Terrestrial Radio Access Network (E-UTRAN) Cell Global Identifier (ECGI) as a global cell IDentifier (ID) of the target cell, in order to perform an alternative handover.
The alternative handover causes extra air signaling overhead and longer handover latency than latency that occurs during normal handover, resulting in degradation of handover performance. In order to avoid the alternative handover causing performance degradation, the LTE system avoids the PCI confusion using a Self Organizing Network (SON) server.
FIG. 1 is a diagram illustrating a conventional PCI management mechanism.
Referring to FIG. 1, a terminal (hereinafter, referred to as a User Equipment (UE)) 103 connected to a serving base station (hereinafter, referred to as an Evolved Node B (eNB)) eNB 1 101a (PCI=1) moves to an area of an eNB 2 101b (PCI=2). The UE 103 reports the PCI (PCI=2) of the eNB 2 101b to the serving base station eNB 1 101a, as denoted by reference number 110. Upon receiving the PCI, the eNB 1 101a searches the NRT for the PCI (PCI=2) received from the UE 103, as denoted by reference number 115. If it is determined that a new eNB is found (i.e. if the PCI does not exist in the NRT), the eNB 1 101a requests the UE 103 for an ECGI corresponding to the PCI 2, as denoted by reference number 120. Upon receiving the request, the UE 103 performs an ECGI acquisition procedure with respect to the eNB corresponding to the PCI (PCI=2) and acquires an Internet Protocol (IP) address based on the acquired ECGI.
The eNB 1 101a sends an X2 SETUP REQUEST message to the eNB 2, as denoted by reference number 125. At this time, the X2 SETUP REQUEST message includes the PCI of the eNB 1 101a. Upon receiving the X2 SETUP REQUEST message, the eNB 2 101b compares the PCI of the eNB 1 101a with PCIs stored in its NRT. At this time, the eNB 2 101b checks whether the PCI of the eNB 1 101a is identical to the PCI of a neighbor eNB 3 101c and reports the PCI confusion to the SON server 107, as denoted by reference number 130.
Next, the eNB 2 101b transmits an X2 SETUP RESPONSE message to the eNB 1 101a, as denoted by reference number 135. At this time, the X2 SETUP RESPONSE message includes the NRT of the eNB 2 101b. Upon receiving the X2 SETUP RESPONSE message, the eNB 1 101a compares its PCI with the PCIs in the NRT transmitted by the eNB 2 101b, as denoted by reference number 140. Upon a determination that the PCI of the eNB 3 101c included in the NRT transmitted by the eNB 2 101b is identical with its PCI, the eNB 1 101a reports the PCI confusion to the SON server 107. As a consequence, the SON server 107 reallocates PCIs, as denoted by reference number 145.
In this PCI confusion detection procedure, the neighbor cell must provide its NRT information. Meanwhile, the LTE standard specifies that the NRT information of the neighbor cell is transmitted through X2 interface. However, most network operators do not want to configure X2 interface among macro and pico base stations in a Heterogeneous Network (HetNet) environment.
Operators do not want configuration to be performed in this manner because, if a macro cell and a plurality of pico cells within the range of the macro cell are connected through X2 interface, an X2 message exchange overload is inevitable. Accordingly, most network operators do not want to configure X2 interface among the macro and pico eNBs.
When no X2 interfaces are configured among the macro and pico eNBs, the PCI confusion may be solved through X2 interfaces among macro eNBs and pico eNBs. However, if there is a plurality of pico eNBs within the range of the macro cell (i.e. if the pico cell PCI confusion occurs within a macro cell), the alternative handover is triggered in the handover attempt between the macro and pico eNBs.